The present invention relates to a magnetic resonance diagnosing apparatus (MRI), and more specifically, to a medical MRI apparatus suitably for measuring a temperature distribution.
Very recently, MRIs may constitute very important diagnosing means for diseases as diagnostic imaging apparatus capable of drawing tissues under superior conditions in conjunction with X-ray CT. Furthermore, MRIs are not only used in diagnosing purposes, but also are developed as such a technique (Interventional MR, namely IVMR) which is applied to guides for catheters and laser fibers when low invasive medical treatments are carried out. As one of these MRI applications, a temperature distribution of a tissue is detected. This MRI application is attractively known as such a means for monitoring curing conditions of a diseased portion in a real time manner while a laser ablation and/or a focused ultrasonic ablation is carried out, during which a tissue of such a diseased portion as tumor and hernia is burned out so as to be cured.
Among parameters for defining a magnetic resonance signal (MR signal), such a parameter indicative of a temperature dependent characteristic involves the spin density xe2x80x9cxcfx81xe2x80x9d, longitudinal relaxation time xe2x80x9cT1xe2x80x9d, the transverse relaxation time xe2x80x9cT2xe2x80x9d, the diffusion coefficient of water, the chemical shift xe2x80x9c94xe2x80x9d of water proton, and the like (see J. C. Hindman, J. Chem. Phys. Volume 44, page 4582, 1966). Among these parameters, it is known that the reliability of chemical shift of water proton is high, in view of less dependent characteristics to the factors except for the temperature.
As a utilization method of a chemical shift, a method for employing a phase map is more effective, since spatial resolution is high and measurement time is short (see JP-A-5-253192 xe2x80x9cA Precise and Fast Temperature Mapping Method Using Water Proton Chemical Shiftxe2x80x9d Y. Ishihara, A. Calderon et al., Abstracts of the Society of Magnetic Resonance Medicine, 11th Annual Meeting, Berlin, p. 4803 (1992)).
This method is performed as follows: That is, while using the sequence having the chemical shift sensitive characteristic such as the gradient echo (GrE) method, a change in the chemical shifts occurred before and after the temperature change is detected as the phase difference of the MR signal. The frequency shift of water proton caused by the temperature is equal to 0.01 ppm/xc2x0 C., and the phase difference xe2x80x9cxcex94xc3x8 is expressed by the following formula (1):
xcex94xcfx86=2xcfx80xc2x7xcex94xcex4xc2x7xcex3Boxc2x7TExe2x80x83xe2x80x83(1)
In this formula (1), symbol xcex94xc3x8 shows the phase difference in the pixel of interest, symbol xcex94xcex4 represents a change in the chemical shifts of water proton in this pixel of interest, symbol xe2x80x9cxcex3xe2x80x9d denotes the gyromagnetic ratio, symbol Bo shows the static magnetic field strength, and symbol TE indicates the echo time. These symbols are similarly applied to the below-mentioned descriptions.
Further, the temperature difference xcex94T is calculated from this phase difference xcex94xc3x8 based upon the following formula (2):                               Δ          ⁢                      xe2x80x83                    ⁢                      T            ⁡                          (                              x                ,                y                            )                                      =                              Δφ            ⁡                          (                              x                ,                y                            )                                            2            ⁢                          π              ·              γ                        ⁢                          xe2x80x83                        ⁢                          Bo              ·              TE              ·              α                                                          (        2        )            
In this formula (2), symbol xe2x80x9cxcex1xe2x80x9d indicates the temperature dependent characteristic of the chemical shift of water proton, i.e., [0.01 ppm/xc2x0 C.]. This symbol is similarly applied to the below-mentioned explanations.
The measuring precision of the temperatures by this method may depend upon both the S/N ratio of the signal and the stability of the hardware, and is on the order of +1xc2x0 C. to xe2x88x921xc2x0 C.
In the conventional phase map formation by employing the GrE method sequence, it is practically difficult to form the phase map within the short time, since the phase encode loop must be repeated along one direction of the space in the 2-dimensional measurement, and the dual phase encode loop must be repeated along two directions of the space in the 3-dimensional measurement. As one example, assuming now that the echo time TE=20 ms, the repetition time TR=30 ms, and the phase encode step number is 64 by way of the highspeed GrE method, approximately 2 seconds are required to form the image. Moreover, in order to execute the slice encode by 16 steps, 32 seconds are required. In a temperature measurement executed in IVMR, a temperature change of a diseased portion, which is caused by a focused ultrasonic medical treatment and the like must be monitored in the real time mode. Also, it is desirable to image several sheets of images per 1 second, and also it is preferable to display a 3-dimensional temperature distribution in combination with these images. However, as previously explained, in accordance with the conventional GrE method, these operations could not be realized.
Therefore, an object of the present invention is to provide such an MRI apparatus capable of forming/displaying a temperature distribution image within a very short time period.
To solve the above-explained problem, in accordance with the present invention, as a radio-frequency magnetic field used to excite water proton, a series of radio-frequency pulses (will be referred to as a xe2x80x9cburst wavexe2x80x9d hereinafter) is employed which is constituted by a plurality of sub-pulses. Also, such a gradient magnetic field echo is produced which owns a higher phase sensitivity characteristic than that of a spin echo. As a result, a phase map can be formed in very high speeds, and also a temperature distribution can be displayed in very high speeds.
It should be understood that as the highspeed imaging sequence with employment of the burst wave, the burst method is known (JP-B-6-34784). An MR image diagnosing apparatus of the present invention is featured by that while a sequence is executed which is made by modifying the sequence of this burst method, a gradient magnetic field echo is produced to which a phase rotation proportional to a chemical shift is applied, and thus, both a phase map and a temperature map may be acquired.
In other words, an MRI apparatus, according to the present invention, is featured by such a magnetic resonance image diagnosing apparatus comprising: magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a radio-frequency (RF) magnetic field in a space where an object under examination is located; detecting means for detecting a magnetic resonance signal produced from the object under examination; means for reconstructing an image based upon the detected magnetic resonance signal; display means for displaying thereon an image; and control means for controlling each of the means, wherein:
the control means executes the below-mentioned sequences, namely:
1) a burst wave is applied as the radio-frequency magnetic field, and at the same time, the gradient magnetic field along at least one direction is applied;
2) a gradient magnetic field in the same direction as that of said gradient magnetic field is applied as a readout-operating gradient magnetic field, and an MR (magnetic resonance) signal is detected as a gradient magnetic field echo;
3) when the burst wave is applied, or the magnetic resonance signal is detected, such a gradient magnetic field which phase-encodes the magnetic resonance signal is applied;
4) the detected magnetic resonance signal is Fourier-transformed, and a phase distribution is calculated based upon both a real part and an imaginary part of complex data of the Fourier-transformed magnetic resonance signal; and
5) an image is constructed from the phase distribution or a temperature distribution obtained from the phase distribution, and the constructed image is displayed on the display means.
In this case, a burst wave will be referred to as xe2x80x9ca series of RF pulsesxe2x80x9d which is constituted by employing a plurality of sub-pulses xe2x80x9cpxe2x80x9d as indicated in FIG. 3A. When a burst wave on a time axis is Fourier-transformed, a series of pulse stream having the same pulse number as that of the burst wave is obtained (see FIG. 3B) on a frequency axis. In this case, it is so assumed that an interval of the sub-pulses which constitute the RF burst on the time axis is equal to xe2x80x9cuxe2x80x9d (seconds) and also an entire length of the pulse stream is equal to xe2x80x9cWxe2x80x9d (seconds), an interval of rectangular waves which constitute the pulse stream on the frequency axis is equal to xe2x80x9c1/uxe2x80x9d(Hz) and a width thereof becomes xe2x80x9c1/Wxe2x80x9d(Hz). Since the burst wave is used to excite a region 301 and the gradient magnetic field is applied thereto in this manner, this comb-shaped region 301 may be excited along the gradient magnetic field direction (namely, x direction in this drawing) as shown in FIG. 3C. Also, echoes whose number is equal to that of the RF pulses may be produced. Since different phase encodes are applied to these echoes, either a 2-dimensional phase distribution or a temperature distribution may be acquired by one shot. As a consequence, while either a phase image or a temperature distribution image is updated in real time in IVMR, these images may be displayed. Since the highspeed inversion of the gradient magnetic field is no longer required in this method, the image formations can be made in higher speeds, as compared with the EPI (Echo Planar Imaging) method capable of forming the 2-dimensional image by one shot.
Also, in accordance with the present invention, since the MR signals are measured as the gradient magnetic field echo which is produced by the diphase-rephase of spins, it is possible to acquire such data having the phase sensitivity characteristic.
In one mode of the present invention, after a burst wave has been applied, a magnetization-inverting radio-frequency magnetic field pulse is applied in combination with a slice-selecting gradient magnetic field. In this case, a gradient magnetic field echo is produced at a different time instant from such a time instant when a spin echo is produced by applying the magnetization inverting radio-frequency magnetic field pulse. Assuming now that a difference between the spin echo producing time instant and the gradation magnetic field echo producing time is xe2x80x9ct0xe2x80x9d, such a phase rotation is applied to a signal, and this phase rotation is proportional to both this time difference xe2x80x9ct0xe2x80x9d and the chemical shift.
In the present invention, only the gradient magnetic field echo may be produced without producing the spin echo. In this case, the effective TE may be set to a long echo time, and also the phase sensitivity can be improved. It should be understood that since the slice selection by employing the magnetization-inverting radio-frequency magnetic field pulse cannot be carried out, this method is suitable for such a 3-dimensional measurement that the phase encode operation is carried out along the slice direction.
Also, in accordance with another mode of the present invention, a gradient magnetic field for a phase encode operation is also applied along the slice direction, the encode steps are repeated as to the slice direction so as to acquire data, so that a 3-dimensional phase distribution is formed. In this case, after the burst wave has been applied, the magnetization-inverting radio-frequency magnetic field pulse may be applied. Alternatively, the magnetization-inverting radio-frequency magnetic field pulse may not be employed.
In the case that the encode steps are repeated, while the frequency of the burst wave is changed every one cycle of the encode steps, different portions which are located in parallel to the read direction are excited, and also the excitating-purpose burst wave may be applied without waiting for the longitudinal magnetization recovery time.
As previously explained, since the burst wave is applied and also the gradient magnetic field along the readout direction is applied, the longitudinal magnetization along the read direction is excited in the comb-teeth shape (stripe shape), so that only a portion of the longitudinal magnetization is excited. As a consequence, when the excitation frequency is shifted and the longitudinal magnetization of the unexcited portion (302 in FIG. 3C) is excited, the subsequent excitation may be carried out after shorter waiting time than the normal TR has passed.
Also, in accordance with the present invention, the temperature distribution may be obtained as follows. That is, the phase distribution measurement is carried out at the different time instants two times, or more. The difference between these phase distributions is calculated, and this phase difference is converted into a temperature change. Then, the temperature distribution may be calculated from this temperature change. A temperature distribution image may be displayed in such a manner that a temperature difference is expressed by a color hue, gradation, or these combinations, while the normal temperature (for example, body temperature of object under examination) is used as a reference. The temperature distribution image may be preferably displayed by being superimposed on a tissue image. As a result, while such a tissue where a temperature change happens to occur is confirmed in colors and the like, the IVMR treatments may be advanced.
As previously explained in detail, in accordance with the present invention, since the imaging sequence employs the burst wave as the excitation-purpose radio-frequency magnetic field and this imaging sequence owns the high phase sensitivity, the 3-dimensional temperature distribution can be displayed in high speeds, and also, the safety aspects as to the IVMR operations under monitoring of MRI can be improved.